1. Field of the Invention
The present invention relates to addressing arrangements within telecommunication switching systems, and more particularly, to the representation of subscribers with multiple appearances of the same directory number within call processing software of telecommunication switching systems.
2. Description of the Related Art
In complex data processing systems, such as telecommunication switches, multiple software units are employed and information is transferred between the units by means of software signals. Each of these signals comprise one or more data words and each software unit is processed by one or more associated processors. Each processor is, in turn, formed in part by a number of different registers which store the data words sent to a software unit in the associated processor. Thus, the maximum number of data words that can be transferred in a software signal is, at least in part, a function of the number of available registers in the processor.
Actions taken within each software unit or module depend upon the data carried by the signal or signals that activate the processing in the particular software module. When a new functionality is added to a complex switching system, the content and the meaning of some of the data words must be changed. It may also be necessary to add new data words in the existing signals. Such a need generates implementation problems in that various specific items of data may "float" (that is, be "used") throughout the complex system in many different software modules. Thus, any change in data word format, such as the addition of additional fields of data, may require the updating of a large portion of the software in each of the different modules resulting in considerable expenditure of time and money. It is also possible for the software signals to become "full", i.e., wholly lacking any space for the addition of new data words or data fields within those signals. In such cases it may be necessary to add entirely new signals in order to carry the additional fields of information similarly requiring modification of the software at substantial expense.
The types of problems that can arise in adding functionality to a data processing system can be seen in telecommunication switching environment. For example, in conventional switching systems which provide plain old telephone service (POTS), there has historically been a one-to-one relationship between the directory number assigned to the subscriber and the telephone instrument or device that subscriber number is assigned to. In such systems, a single data word is used as an identity throughout the software modules of such a system to identify which subscriber interface, and thus which subscriber number, a certain activity or function is related to.
Referring to FIG. 1, for example, there is shown a block diagram depicting a data structure for a subscriber interface and subscriber number in a conventional telecommunications switching system. In FIG. 1, it can be seen that a set of subscriber data files 2 is used to store information regarding a subscriber's properties and features. A single pointer 4 to these files is used tc access subscriber data from throughout the overall telecommunication switching system. This pointer 4, a subscriber data file pointer, plays a central role in traffic handling and in the administration of subscriber data.
When a subscriber goes off-hook, the subscriber data file pointer 4 is fetched from the line interface files to establish a line access. This pointer 4 is then used during the call to identify the call and to fetch data from the subscriber data files 2 as needed. Data which may be fetched included subscriber features, signalling schemes to be used, charging information and other vital information for call handling and administration.
When a call is terminated to a subscriber in conventional telecommunication switching systems, the analysis of the digits leads to the subscriber data file pointer 4. With this pointer 4, a line interface file pointer 6 may be accessed and activated. Both pointers cooperate to identify the subscriber access.
The above-described conventional system functioned adequately as long as a one-to-one relationship exists between the subscriber line interface and the subscriber number. When such a relationship does no longer exist, the system does not work. For example, in the integrated services digital network (ISDN) line identity is no longer sufficient alone to identify the calling or called party since a single directory number may be associated with multiple terminals or subscribers. To identify these subscribers, ISDN uses the concept of sub-addressing within ISDN messages sent through the telecommunications switch, as referred to U.S. Pat. No. 5,008,884 to Yazawa et al., which renders the subscriber data file pointer and/or a line interface pointer no longer sufficient to identify a particular terminal within the call handling software of the switch itself. More particularly, an ISDN basic access line may have up to eight (8) terminals connected, each with a different set of service categories and features. An electronic key telephone system (EKTS) terminal, a subset of ISDN telephone service, may have up to one hundred twenty eight (128) subscribers connected to it. In other words, for the same line interface record, many records in the subscriber data file are necessary in order to adequately define and specify subscriber information. On the other hand, in a multiple directory number group, the same subscriber number may be shared by up to five hundred twelve (512) line interfaces. This results in one subscriber data file record for many line interface records. Clearly, prior art mechanisms and subscriber identifications are inadequate in modern telephone switching systems.
The technique of using both a subscriber identification and sub-address field as provided in ISDN, functions adequately where means are provided external to the switch for decoding the ISDN message containing the sub-address. Such decoding is performed within the individual ISDN terminals as part of their communication with other terminals via the ISDN protocol. Such sub-addressing within the ISDN protocol is not recognized by the switching system itself as terminal identity, but only carried through the switch as a data message between line access connections. As pointed out above, today the vast majority of telecommunication switches providing POTS service rely upon a subscriber pointer SCp to identify the subscriber within the software signals communicated between different call processing software modules. The addition of a separate data field for sub-addressing different terminals having the same directory number would require essentially rewriting all of the software modules to accommodate a new field of data within their system of signals. Such rewriting of software is inordinately expensive and requires the modification of numerous modules which have no need whatsoever to distinguish between separate individual terminals assigned to the same directory number.